Coatings by means of electrolytic baths present many drawbacks at a technical and environmental level. The control systems required for the processes of electrolytic baths are very strict and the processes are very complex especially with non-conductive substrates.
In the case of chrome-plating, the chromium coatings by electrolytic baths are used as protective layers against corrosion and as a decorative finish. In general, these coatings are carried out in chemical baths formulated with hexavalent chromium, a compound classified as carcinogenic.
Recently, the industry has begun to evolve toward trivalent chromium as an acceptable alternative from the point of view of environmental protection. At the present time, this alternative has several limitations such as a) low resistance of the coating to corrosion, b) finishes of poor quality, and c) formation of hexavalent chromium during the process through anodic oxidation.
Moreover, in the conventional methods of coating by electrolytic baths, plastics are not conductive. Consequently, they must be subjected to complex processes of pre-treatment to be able to receive the technical coatings that confer the properties upon them. This preparation is complex and it contemplates the use of highly toxic and difficult to control products.
To obtain an article with a finish of electrolytic chromium, different layers must be applied to obtain the desired color and resistance to corrosion. For chromium deposition to obtain the desired color and gloss, it is necessary to apply onto the thermoplastic, metallic or a metallic alloy material successive layers of semi-gloss and gloss nickel. Also, depending on the articles treated, a layer should be applied to promote adhesion of the copper. The introduction of intermediate metallic layers increases the possibility of corrosion of the piece with the least defect in the final layer of chromium.
Furthermore, the use of electrolytic baths requires special equipment. The baths are very sensitive and require a separate control unit for each vat because each bath has different temperature, filtration, agitation and amperage characteristics.
Although improvements have been introduced in the process of trivalent chrome-plating with regard to color, appearance, process control, etc., it has not been possible to obtain a deposit with the same surface characteristics as obtained with the process of chrome-plating based on hexavalent chromium.
Electrodeposition with trivalent chromium is problematic because of low stability in the baths, resulting from the formation of hexavalent chromium through anodic oxidation. Additionally, trivalent chromium baths as proposed in U.S. Pat. No. 5,560,815 use an adapted electrode, thereby managing to substantially reduce the formation of hexavalent chromium. However, it has not been possible to completely eliminate this compound.
Another form of providing a finish of decorative metallic appearance, protective against corrosion comprises application of a coating by methods of physical vapor deposition (PVD) with prior application of electrolytic coatings which perform the function of masking substrate surface defects. The following patents describe application of a coating by a vapor phase method to articles previously coated by means of electrodeposition: U.S. Pat. No. 5,814,415; U.S. Pat. No. 5,667,904; U.S. Pat. No. 5,626,972; U.S. Pat. No. 5,948,548; U.S. Pat. No. 6,221,231; U.S. Pat. No. 6,106,958; U.S. Pat. No. 6,245,435; U.S. Pat. No. 5,759,677; EP 928343.
Another form of providing a metallic finish (chromium) (as in published U.S. Patent Application 2002/0170460), is to use an organic layer to smooth the surface of the layer on which a multilayer of Cr—Ni is applied to improve adhesion and subsequently, the final layer of chromium. The application of both layers, Cr—Ni and chromium, is carried out by PVD. Finally, an organic layer is applied to protect the PVD multilayer.
In this process, the use of electrolytic chromium is suppressed, but it requires the application of two organic layers; one to smooth the surface of the article and another one to protect the Cr—Ni/Cr multilayer against corrosion due to the presence of the nickel. The use of multilayer metallic coatings introducing nickel, increases susceptibility to corrosion of the articles, complicates the process of coating the different articles, and excludes arc PVD as the coating technique because of the magnetic properties of nickel.
On the other hand, there is the possibility of using a polymeric layer in substitution of the electrolytic metallic layers and deposition of the ensuing chromium coating by PVD. An example of this coating is described in U.S. Pat. No. 6,652,988, which refers to an article having a coating with decorative appearance, resistant to friction, wear and corrosion. The coating comprises a polymeric layer product of the reaction of an epoxy/urethane resin and polyamide or an epoxy/urethane resin in organic solvents, and a decorative coating formed by at least one layer obtained by vapor phase deposition. The layer comprises a chromium, a chromium compound, a refractory metal compound, or a refractory metallic alloy compound layer.
This coating avoids the need for application of electrolytic layers, but requires employment of urethane resins which contain isocyanates. Isocyanates are hazardous, presenting health risks during application, resulting from inhalation of isocyanate droplets which have not reacted. In addition to this serious drawback, this procedure is restricted by the type of substrate to be coated. The curing temperature of resins based on epoxy/urethane is relatively high, impeding its use on certain articles fabricated in materials of low melting point, particularly thermoplastics.
Thus, although diverse procedures are known in the state of the art to apply decorative and protective coatings on thermoplastics, metallic or metallic alloy articles, the need continues for alternative procedures which, as well as allowing a coating to be obtained with a metallic or ceramic finish resistant to chemical and mechanical corrosion, assure the absence in the process and in the coated article of compounds dangerous to health and which, in addition, allow working with a broader range of substrates without temperature restrictions. Examples of such dangerous compounds being isocyanates or, in the case of chromium coatings, hexavalent or trivalent chromium.
The coating procedure of the present invention, achieves coatings with metallic or ceramic finish avoiding the formation of isocyanates and allowing the coating of articles fabricated with materials of low melting point. Moreover, using photosensitive resins, UV curing, being instantaneous, allows shorter processes and, is therefore, more economical at the industrial level, thereby avoiding possible chemical attacks on the sensitive substrates.
Likewise, for coatings with a finish of electrolytic chromium, based on metallic chromium, metallic chromium is deposited without the formation of hexavalent chromium or trivalent chromium in the process.